This application describes studies aimed at elucidating the mechanism by which the HIV-1 Nef protein induces CD4 endocytosis. The conservation of nef in primate lentiviruses, as well as experiments done in the SIV system, indicate the it is essential for the persistence of these viruses in the infected host, and for the pathogenesis of AIDS. Yet, the mechanisms of Nef action remain a mystery, hampering the rational development of therapies which could target this viral protein. Work performed in the laboratory of Dr. Trono, the U.S. Principal Investigator, has demonstrated that nef triggers the rapid internalization of CD4, which is the main HIV receptor, but also plays a major role in immune recognition. This application proposes to further dissect this phenomenon, through an intensified collaboration between Dr. Trono's group and that headed by Dr. Jean-Louis Carpentier, from the Department of Morphology at the University of Geneva, Switzerland. Dr. Carpentier's expertise int he field of endocytosis will allow: 1. To perform an ultrastructural analysis of CD4 endocytosis, in the presence or the absence of Nef. 2. To analyze Nef- mediated CD4 internalization in the context of a cell-free assay for endocytosis, recently developed in his laboratory. It is strongly hoped that these studies will lead to major breakthroughs in the understanding of Nef function, as well as of CD4 metabolism. Ultimately, new therapeutic approaches for HIV might be suggested by these experiments.